1. Field of the Invention
The present invention relates to an extrusion die for making multi-hole flat tube suitable for making various types of aluminum heat exchangers, for example. This application is based on Japanese Patent Application Nos. Hei 11-277613, 11-227614, and 2000-155342, and the contents of which are incorporated herein by reference.
2. Description of the Related Art
Extrusion fabrication of various metals is a known technology for producing component parts for various electrical devices and automobiles. Extrusion fabrication is widely used in making aluminum tubing for various heat exchangers, such as evaporator, condenser, radiator and others for use in automobile air-conditioners because of the excellent hot working properties of aluminum. An extrusion die is used in practice to perform extrusion fabrication.
FIGS. 17xcx9c21 show some examples of known extrusion dies (refer to a Japanese Patent Application, First Publication No. Hei. 7-124634). A typical die shown is comprised by a male die 31 has an external appearance of a rectangular plate (refer to FIG. 17), and a female die 41 has an external appearance of a cylindrical column (refer to FIGS. 18, 19).
As shown in FIG. 17, the male die 31 having a specific plate thickness is made of a high-speed steel or a hot worked die steel, and in a central section of its end surface 32, a step section 33 is protruded from the surface. From the center of the step section 33 extends a protrusion section 34 containing a series of protrusions resembling comb teeth. Also, flow-in sections 36 sloping down toward the protrusion section 34 are formed on the wide surfaces 35 sandwiching the end surface 32. The both sides of the wide surfaces 35 having the flow-in sections 36 are referred to as coupling sections 37 of a uniform plate thickness.
On the other hand, as shown in FIGS. 18, 19, the female die 41 is comprised by a female die body 42 made of a die steel, and an insert member 43 made of the material harder than that of the female die body 42 such as a high-speed steel or a hard metal. The female die body 42 has an external appearance of cylindrical bar, and a radially extending channel section 45 of a given width is formed on an end surface 44 opposing the male die 31. A rectangular recess 47 is formed in the center of the bottom surface 46 of the channel section 45 for inserting the insert member 43.
The insert member 43 is formed in such a way that when it is coupled inside the recess 47, its upper surface is coplanar with the bottom surface 46 of the channel section 45, and in the center, a through-hole 48 that extends in an orthogonal direction to the channel 45 is formed, as shown in FIGS. 18, 19. The through-hole 48 is comprised by a die cavity 49 that has an elliptic shape and positioned at the top surface of the insert member 43 for inserting the protrusion section 34 to fabricate an aluminum material (billet) into a desired shape in the space formed between itself and the protrusion section 34, and an exit opening 50 that opens at the downstream surface of the insert member 43 and is shaped in such a way that the width of the exit opening is larger than the width of the die cavity 49. The insert piece 43 is shrink fitted to firmly engage with the recess 47.
Also, grooved channels 51 are formed on the two end surfaces 44 on both sides of the channel section 45 of the female die body 42, whose depth is shallower than that of the channel section 45, to extend in an orthogonal direction to the channel section 45. On the opposite end surface 52 of the female die body 42, a first hole section 53 whose entry end communicates with the through-hole 48 and whose exit end opens at the end surface 52 of the female die body 42 for discharging the extrusions, and a second hole section 54 cut out so as to cross the first hole section 53, and whose entry end opens at the bottom surface of the recess 47 and the exit end opens at the end surface 52.
The extrusion die is used by forming an integral die by locating the protrusion section 34 of the male die 31 within the die cavity 49 of the female die 41, and engaging the coupling section 37 of the male die 31 into the grooved channel 51 of the female die 41, and coupling the step section 33 of the male die 31 with the channel section 45 of the female die 41.
The extrusion die comprised by the male die 31 and the female die 41 is inserted into the through-hole of the die-holder that serves as a flow path of the metal, and is fixed therein, and the aluminum billet inserted into a billet hole of the container communicating with the through-hole of the die holder, is pressed towards the extrusion die by a stem of an extrusion press which is omitted in the Figure. The billet being extruded flows into a billet flow passage formed between the two wide surfaces 35 and the inner wall surface of the through-hole of the die-holder to the space formed between the protrusion section 34 of the male die 31 and the die cavity 49 of the female die 41, and in passing through the space formed between the die cavity 49 and the protrusion section 34, the multi-hole flat tube Ca such as the one shown in FIG. 20 is produced.
When the billet flows into the flow passage of the extrusion die, a high-temperature and high-pressure material impinges directly on the protrusion section 34 of the male die 31 to apply a high pressure so that the protrusion section 34 is rapidly worn out. For this reason, it is necessary to change the male die 31 most frequently, which results in a problem of high die cost. This problem is most severe when making the multi-hole flat tube Ca having many holes such as the one shown in FIG. 20, because of thinner size of the protrusion section 34 of the male die 31 resulting in low wear resistance.
To resolve such a problem, recent die development efforts resulted in an introduction of a two-piece construction of the core section that includes the protrusion section 34 at the tip of the male die 31 as indicated by 2-dot line in FIG. 17, for example. A core 31a is made separately of a wear resistant hard metal, and the other part of the male die body 31b is made of a regular die steel (refer, for example, to a Japanese Patent Application, First Publication, No. Hei 9-155438).
However, in the above process of making separate members, i.e., male die body 31b and a core 31a made of a hard metal or a high-speed steel, a difficulty is experienced in making the protrusion sections 34 at the tip of the core 31a shown in FIG. 21. This is because the hardness of the material itself is very high and high dimensional accuracy is required in making such fine parts, such parts can only be made currently by a normal discharge machining process using electrode plates or wire electrical discharge machining.
A disadvantage of such machining processes based on normal discharge machining based on electrode plates or wires is that because the core 31 a is made of a hard metal or high-speed alloys of very high hardness, although wear resistance is improved to a degree, the fabricated product is extremely vulnerable to chipping of the protrusion section 34.
The present inventors have undertaken detailed study of the protrusion section 34 of the core 31a made by the normal discharge machining or wire electrical discharge machining using scanning electron microscope. It was found that a molten abnormal layer that contains bumpy surface irregularities is formed on the surface of the protrusion section 34, and surface chipping and micro-flaking at the edge portion of the protrusion section 34 are experienced. It was thought that these are one of the reasons for making the protrusion section 34 susceptible to breakage.
That is, the nature of discharge machining is such that an electrode (plate, wire and the like) is positioned at a distance from an object to be fabricated so as to cause arc discharge between the electrode and the workpiece to produce melting and vaporizing of the material near the discharge point and blowing of the debris by explosive action of arc discharge. The present inventors reasoned that such violent high temperature melting by discharge machining would tend to produce defects such as the molten abnormal layer or corroded layer of low mechanical strength, as well as concentration of arc discharge on edges of the protrusion section 34 that are particularly susceptible. Such phenomena would cause a loss of strength of the protrusion section 34.
It is therefore an object of the present invention to provide an extrusion die having a superior wear resistance and durable protrusion sections.
The features of the present extrusion die to resolve the problems in the existing dies are provided in an extrusion die comprised by a male die having a plurality of protrusion sections separated at a given distance and a female die having a die cavity for inserting the protrusion sections, so that a die assembly prepared by linking coupling sections of the male die with coupling sections of the female die is installed in an extrusion press in such a way that a billet material is pushed through spaces formed between the die cavity and the protrusion sections to produce a multi-hole flat tube, wherein
the male die is comprised by a male die body and a core member having the plurality of protrusion sections firmly locked in a central region of the male die body; wherein
the core member is comprised by individually fabricated protruding rod members, comprising the protrusion sections and associated base sections, to be locked-in firmly with the male die body, that extend upstream with respect to a material flow.
In the present invention, the core is made up by a plurality of individually prepared protruding rod members so that the structure of the core is much simpler than the design of the conventional core so that they can be fabricated not using the discharge machining as much as possible but by using other fabrication methods such as grinding and polishing. Such simple mechanical processing methods do not produce the molten abnormal layer (bumpy surface) or chips and micro-flaking are less prone to be generated on the surface. For these reasons, durability of the core is increased significantly.
A second aspect of the present die is that the protrusion section formed at a downstream end on each of the protruding rod member serve as a shaping section to extrude the billet material into a given shape by contacting the billet material and the base section on an upstream end has a locking notch to prevent the protruding rod member to shift in an axial direction.
In the present invention, the locking section fabricated in the base section is engaged with the male die body when locking the protruding rod member to the male die body, thereby restricting any shift of the protruding rod members in the axial direction. Although the protruding rod members are under a great pressure when extruding the material caused by frictional forces generated by the flowing material, there is no danger of the protruding rod member shifting in the flow direction because the protruding rod members are locked-in by the male die body.
Another aspect of the invention is that the protruding rod member is fabricated by grinding at least those locations that contact the material.
In the present invention, the part that is most vulnerable to wear due to high load on the core is fabricated by grinding, and such grinding process can produce flat surfaces while suppressing surface roughness to produce smooth flat surfaces. Therefore, if it is desired to apply a film of coating on the areas that contact the material, it is possible to increase the bonding strength between the mother base of the core and the coating film, thereby increasing the durability of the core.
Still another aspect is that the protruding rod member is fabricated by polishing at least those locations that contact the material.
In the present invention, the part that is most vulnerable parts of the core to wear due to high load on the core are fabricated by polishing, and such grinding process can produce flat surfaces while suppressing surface roughness even more to produce smooth flat surfaces. Therefore, if it is desired to apply a film of coating on the areas that contact the material, it is possible to increase the bonding strength between the mother base of the core and the coating film even further, thereby increasing the durability of the core even further.
Still another aspect is that the protruding rod member is fabricated by first grinding at least those locations that contact the material, followed by polishing. Because the most vulnerable parts of the core to wear due to high load on the core are fabricated by grinding first followed by polishing, flat surfaces are produced quickly by grinding while increasing the precision of surface finish. Therefore, it is possible to satisfy both requirements of fabrication speed and fabrication precision.
Still another aspect is that the protruding rod member is fabricated by first electrical discharge machining or wire discharge machining and the like at least those locations that contact the material, followed by polishing. Because the most vulnerable parts of the core to wear due to high load on the core are fabricated by the electrical discharge machining first followed by polishing, fabrication speed is increased by the electrical discharge machining while increasing the precision of surface finish by polishing. Therefore, it is possible to satisfy both requirements of fabrication speed and fabrication precision.
Final aspect is that a protruding rod member fabricated by any of the methods proposed in the present invention has a finely serrated section fabricated along the axial direction at least around external tip regions of the protruding rod member.
Accordingly, because a serrated section is provided at least on the downstream tip section of the protrusion section, serrated lines are produced on the interior wall of the multi-hole flat tube produced from the present extrusion die, thereby increasing heat transfer area and generating turbulence in the stream flowing in the tubing so that the heat transfer efficiency of the heat exchanger is improved significantly.
As an overall summary of the present invention, it may be noted that the core is made of a number of individual protruding rod members, and the structure of the rod members is much simpler than that of the conventional core, method of fabrication is not limited the electrical discharge machining, and other methods such as grinding and/or polishing may be utilized. Simple mechanical fabrication such as grinding is able to finish the surface without creating many surface bumps compared with the electrical discharge machining process so that if it is desired to apply a wear resistant coating on the surfaces of the rod members to produce a durable core, with a titanium group of coatings for example, a strong bonding can be produced at the interface between the coating film and the base material. Also, simple mechanical fabrication methods do not produce the molten abnormal layer or chips, and micro-flaking are less prone to be generated on the surface, thereby enabling to increase the durability of the core significantly.